One in five Americans have spent $500 or more on a purchase without their partner's knowledge, according to a CreditCards.com report released Wednesday. And, bucking the widespread perception, men are much more likely to have done so than their wives. Twenty-six percent of men have spent more than $500 without notifying their significant other versus just 14 percent of women.

It's not just purchases that many are keeping secret. Approximately 7.2 million Americans (4.4 million men and 2.8 million women) have hidden a bank or credit card account from their live-in spouse or partner, the report found.

"This is a big deal both in terms of your finances and in terms of your relationship," Matt Schulz, CreditCards.com's senior industry analyst, told CNBC. "Anytime you have this sort of secret being hidden, it naturally begs the question of what else is being hidden."

Financial planners note that financial deceptions are particularly hard to recover from as they often indicate larger problems—whether it's overspending or more insidious habits like gambling—and are seen as a breach of trust."It is a kind of betrayal, and betrayal is what can start the division in a marriage that can end in a split," said Donna M. Phelan, a financial advisor and author of "Women, Money and Prosperity: A Sister's Perspective on How to Retire Well."

Money is an oft-cited cause of divorce. A 2013 Kansas State University study using longitudinal data from more than 4,500 couples found "arguments about money was by far the top predictor of divorce." And a separate surveyof nearly 200 Certified Divorce Financial Analystscited money as the third leading cause of divorce (behind "basic incompatibility" and "infidelity").

"If a couple argues about money, they're more likely to be hiding assets or spending, and if they're hiding those, they're more likely to get a divorce," said Stacy Francis, president and CEO of Francis Financial and a Certified Divorce Financial Analyst."The deceptions create a fissure within the relationship that's hard to repair."

But there is hope. In a 2010 National Endowment for Financial Education survey conducted by Harris Interactive, 67 percent of those who reported that financial deceptions affected their current or past relationships said it caused arguments, but only 16 percent said it ultimately resulted in divorce.

When, and whether, the guilty party comes clean and how the couple deals with the indiscretion can make the difference between divorce and reconciliation. But regular and candid communication can prevent such deceptions in first place, said Francis. She recommends setting aside a night once a month to check-in with each other about finances—a "financial date night." Using an online budget tool like Mint.com can also help keep you honest.

"The main takeaway from all of this is in terms of relationships, whether financial or romantic, is that honesty is hard to beat," added Schulz. "If you're keeping these sort of secrets you're making things harder on yourself and setting yourself up for trouble."